1. Field of the Invention
The invention relates to dewatering methods and agents used in these methods, especially those methods and agents used for dewatering sludges.
2. Background Art
Dewatering of various types of sludges, such as mineral sludges generated by phosphatic ore mining, has been a subject of intense research due to the economic and environmental impact of disposing of the sludges or, alternatively, of recovering the solids from the sludges by dewatering processes. For instance, the mining and processing of phosphatic ores (apatites) in Florida generates an estimated 100,000 tons of phosphatic waste clay solid suspensions each day. Such suspensions, which are generated as sludge-like slurries, contain 3 to 10% solids, and have historically been pumped to large, above-ground holding ponds where water is decanted through spillways as the solids slowly consolidate under the impact of gravity to a 15-18% solids level. At this solids level the ponds slowly dehydrate and form a crust on their surface which retards further surface evaporation. Without additional physical efforts to dewater the mass, it then may take several decades to consolidate the crusted pond to a solids content of 25-35%. Because as much as 60% of the land surface in a mining area is covered with these ponds, they represent a considerable economic penalty to the mining industry and preclude the profitable use of thousands of acres of central Florida land. This conventional practice also ties up millions of gallons of water for years, losing much of the retained water to evaporation. The severe economic costs of these traditional practices of handling waste clays have thus prompted the mining industry to experiment with methods that dewater the clays more rapidly.
Several dewatering methods are used to obtain a more rapid consolidation. Such procedures have been extensively described in the literature and include (a) sand spraying, (b) dredge-mix, (c) sand-clay mix, and (d) sand-clay flocculation and settling. The most economical methods include the sand-clay mix and sand-clay flocculation methods. In the sand-clay mix method, clays are settled for about a year in a large pond until they reach 15 to 18% solids. The clays are then dredged and mixed with (flotation) tailings in a ratio which, on further consolidation, yield a stable, arable soil. However, the dredging and mixing process may require another three to four years.
The sand-clay flocculation process, known as the Estach process, involves the flocculation of the sand and clay. The mixture is processed through a rake type thickener, mixed with additional sand and then pumped into a disposal area. The 12% slurry consolidates rapidly to about 30% solids in the first year and reaches the 40% level in the next three years.
As indicated above, both of these methods still require years before a mined out area can be returned to productive use. Even further, actual practice indicates that there is no universal method due to site-specifics in regard to types of clays, location of settling areas, economics, etc.
More effective dewatering methods are described in the U.S. Bureau of Mines Reports of Investigations, e.g., Investigation Nos. 8611 (Large-Scale Dewatering of Phosphatic Clay Waste from Central Florida), 8349 (Flocculation Dewatering of Florida Phosphatic Clay Wastes), and others. The investigators in those reports used several polymer flocculating agents, including polyethylene oxide (PEO) polymers and polyacrylamides. The investigators found that PEO polymer of about eight million molecular weight alone was effective as a dewatering agent. They further found that PEO polymer alone was as effective as PEO polymer and polyacrylamides combined, and was more effective than the polyacrylamides alone. See for example, U.S. Pat. No. 4,931,190 to Laros. However, while rapid dewatering is achieved using these methods, the PEO polymer is quite expensive and dewatering after 90 days still does not reach a 30% solids level.
The method described in the above-mentioned '190 patent to Laros also employs PEO polymer, but employs it in combination with a cheaper polymer such as anionic polyacrylamide flocculating agents. By employing this reportedly synergistic combination of flocculating agents in conjunction with a sand slurry, Laros reportedly is able to consolidate a sludge containing 3 to 5% solids by weight into a filter cake having an excess of 40% solids. However, Laros still requires the use of expensive PEO polymer.
Further, PEO polymer is specific in regards to the type of clays for which it will induce flocculation. Because the type of clay varies from mine site to mine site, certain sites may not be able to employ PEO polymer in their dewatering processes.
Polymer flocculating agents have also been employed in other fields of dewatering. For example, PEO polymer and acrylamide polymers have been employed in a method of dewatering organic sludges. See U.S. Pat. No. 4,710,298 to Noda et al. Noda et al.'s method employs one of these polymers with a fibrous material which prior to being added to the sludge is pressed or watered to a specific gravity of at least 0.3 g/cm.sup.3. However, the method described therein cannot be economically and efficiently used to dewater sludges containing mineral solids such as phosphatic wastes.
There also has been interest in improving the methods for dewatering very fine, e.g., less than one micron, slow-settling, tailings from kaolin clay beneficiation plants. Kaolin clay, which is primarily, e.g., kalonite, is mined and beneficiated to produce kaolin particles of a quality and size suitable for fillers, filtering agents and fiber reinforcing agents in paints and plastics. A typical kaolin beneficiation process is disclosed in B. K. Asdell, "Wet Processing of Kaolin" Society of Mining Engineers, December 1967, pp 467-474. During beneficiation, kaolin is dispersed, degritted, e.g., to remove coarse sand and mica, and classified according to particle size. The clay is classified by, for example, centrifugation, flocculation, filtration, redispersion, magnetic separation, or vacuum filtration, followed by several other unit operations which are conducted to separate the product to required specifications. The clay not classified and used, i.e., tailings, are then discarded.
A current practice in clay beneficiation plants is to pump tailings (at about 20% solids) to an impounded settling area where these fine particles settle under gravity. As with settling phosphatic ores, decades likely are required to consolidate the tailings to a solid content high enough to reclaim the area. Certainly, the water remaining with the clay, as well as fast recovery of the land and the clays, are of economic and environmental value. Flocculation followed by mechanical dewatering has been used in the past to accelerate consolidation of kaolin tailings. Such processes, however, are generally uneconomical due to the low capacity of conventional dewatering equipment.
Tailings from aluminum production plants present similar economic and environmental issues. Conventional aluminum production employs the Bayer process and begins by digesting bauxite ores with caustic soda at high temperature and pressures. A description and illustration of the Bayer process is found in a paper which Sankey et al. presented at the 111th AIME annual meeting in Dallas, Tex. on Feb. 16, 1982. The digestion process results in soluble sodium aluminate and a residue known as "red mud." The aluminate liquor then is separated from red mud residue by primary liquor decantation and counter current decantation (CCD) washing. The latter process consolidates the red mud so that the red mud can be returned to the initial processing stages for further decantation and recovery of caustic. Once aluminate is sufficiently extracted from the red mud, the red mud residue tailings from a final CCD washing step is dewatered, as far as possible, using either deep thickeners, super thickeners or vacuum filtration. The dewatered tailings then are disposed into a tailing holding area for lengthy consolidation periods.